I either take pictures or make illustrations myself, or get public domain images from the Wikimedia Commons. I've commissioned an artist once, and while it turned out ok, I'm not sure I want to do it on another massive flop.

The only drawback to using public domain images is that you can't always find exactly what you're looking for.

See, I am good at taking images and making art from them. Combining elements, adjusting color, and so on. I am not any good at taking a blank page and drawing something (or painting, connect the dots, or etc). So I poke around the better image libraries (typically not free) and create form that. My avatar here is an example of a logo I built that way. (I do pay someone else currently to do my book covers)

When I had the time, I was pretty good capturing images with my DSLR, but something had to give in my schedule, and that went. Some day will return to it. Someday....